Previous coaxial thermistor bolometers, of which examples include Hewlett-Packard model 478 and Weinschel (or TEGAM) M1110, used “bead” thermistors, which were hung from fine wires in gaps in complex three-dimensional structures that formed the center conductor of a coaxial airline. Other published bolometers placed the resistive element radially between the center conductor and outer conductor of a coaxial airline. When the termination was a thermistor, it was again hung from a fine wire. In one published concept, a nickel barreter was constructed radially using a thin film of nickel on a substrate.
In all of these concepts, the arrangement of the temperature-sensitive element required a sharp step in the conductor. Bead thermistors hanging on fine wire result in a step in width of the center conductor from, typically, many tens of thousandths of an inch, to under one thousandth of an inch. The resulting inductive discontinuity resulted in unsatisfactory reflection at higher frequencies.
Further, the use of fine wire resulted in a structure that is difficult to assemble and easy to damage. Use of metal barreters in the case of thin-film terminations are easily damaged with excess power.
Accordingly, there is a long felt need in the art for a coaxial thermistor bolometer that eliminates the reflection due to the sharp discontinuity between the waveguide and fine wire of the thermistor, while improving robustness and manufacturability by addressing the mechanical fragility of the wire. Specifically, a bolometer structure is needed wherein the structure of the waveguide leading to the thermistor can be well matched with a new thermistor design that does not depend on fine wires for thermal isolation.